It may however transpire, following a fault or an incident, that the pressurization of the airplane can no longer be maintained at an acceptable level. A regulatory procedure then compels the pilot to take the airplane down, as rapidly as possible, to a breathable altitude of 10 000 feet (about 3000 meters). This procedure is called an emergency descent.
In this case, the crew is responsible for the various tasks related to the initiation of the descent, as well as to the management of the descent until leveling off at low altitude.
It may nevertheless transpire, although very rarely, that the crew is no longer in a position to enter the commands required for piloting the airplane, for example in the case of a pressurization fault that has caused the crew to lose consciousness. The airplane is in this case left to itself, although it is absolutely necessary to perform an emergency descent. If, in such a situation, the automatic pilot is engaged, the flight is continued automatically until the fuel reserves are totally exhausted, following which the airplane drops.
Other circumstances such as the shattering of an engine causing a hole in the structure of the airplane for example may lead to the same situation. The inability of the crew to pilot the airplane in such circumstances may therefore have catastrophic consequences.
To remedy this type of situation, an automatic piloting system is known which, when it is engaged, makes it possible to carry out the emergency descent in an automatic manner, that is to say without requiring the aid of a pilot. Moreover, the engagement of an automatic emergency descent such as this can be carried out, either manually by the pilot, or also in an automatic manner. A mode of automatic engagement of a descent function is based, generally, on the absence of reaction of the crew to an alert which indicates an excessive altitude of the cabin. To detect an excessive altitude, the pressure of the air inside the cabin is measured, and an alert is emitted when this pressure is below a predetermined pressure threshold.
This standard solution exhibits the drawback of triggering the emergency descent function only when the pressure of the air in the cabin falls to said pressure threshold. However, so as not to cause untimely triggerings, it is necessary for this pressure threshold not to be too high. However, there may be a relatively significant duration between the occurrence of a pressurization problem and the attaining of this pressure threshold. And it is known that the longer the exposure of an organism to an insufficient pressure, the greater the risk of hypoxia. Consequently, this standard solution for automatically triggering an emergency descent is not completely satisfactory.